This invention relates generally to a liquid injection cooling arrangement for a rotary compressor. More specifically, the present invention is directed to a liquid refrigerant injection arrangement wherein the liquid refrigerant line is connected from the high pressure side of a refrigeration system to a liquid refrigerant inlet path in the compression cylinder. The inlet path for the liquid refrigerant is routed through the compression cylinder and is provided with an orifice leading into the compression bore. The inlet path decreases in cross-section as it leads from the liquid injection line to the orifice.
Liquid injection methods have been utilized in prior art rotary compressors to reduce the temperatures of the compressor motor windings and the lubricating oil. This has been accomplished by providing liquid refrigerant from the condenser and by using capillary tubes externally of the compressor to provide the necessary pressure drop from the condenser to the compressor cylinder. When the compressor roller exposes the liquid injection aperture in the compressor bore to higher pressures, the refrigerant within the liquid injection aperture and within the path leading from the aperture to the capillary tube is compressed. Then, when the roller revolves further to expel the compressed refrigerant and, thus, generates a lower pressure in the compressor bore permitting refrigerant to flow into the bore from the suction inlet, the refrigerant in the liquid injection aperture and the path leading from the aperture to the capillary tube expands. This cyclical compression and re-expansion of the refrigerant requires work. Thus, because this work is provided by the compressor motor, the compressor overall efficiency is decreased by this prior art method of liquid injection.
In the past, attempts have made to eliminate the above-described lost work of compression and re-expansion by locating the aperture which conducts liquid refrigerant into the compression bore so that the aperture will always be closed by the roller prior to the time at which the pressure within the compression bore becomes greater than the pressure of the refrigerant within the aperture. However, because the pressures of refrigeration systems vary with various atmospheric and loading conditions, the aperture leading into the compression bore must be located within the compression cylinder so that it will be closed prior to the increase of pressure within the bore in excess of the pressure within the aperture under a variety of atmospheric and loading conditions. Consequently, the amount of cooling provided by the liquid injection system is severely limited because refrigerant is introduced into the compression bore over a shorter period of time. This is due to the location of the aperture at a point in the compressor cylinder so as to prevent compression and re-expansion under all atmospheric and load conditions. In other words, under certain atmospheric and loading conditions, the pressure within the compression bore does not become greater than the pressure of the refrigerant in the aperture until the roller has passed substantially beyond the aperture location. Thus, under those conditions, liquid refrigerant insertion cooling which could have been efficiently provided to the motor windings and the lubricating oil is prevented from occurring. The rotary compressor motor, therefore, runs at a higher temperature and the overall efficiency of the refrigeration system is decreased.
Another condition which can vary the pressure of the refrigerant in the aperture to assure that the pressure within the cylinder bore does not exceed the pressure of the refrigerant within the liquid refrigerant injection aperture, is the pressure drop in the liquid refrigerant line leading from the high pressure side of the refrigeration system to the liquid refrigerant injection aperture. Depending on the length, diameter and the interior surface of the liquid refrigerant line, the pressure delivered to the liquid refrigerant injection aperture leading into the compression bore will vary. Further, because during the manufacture of refrigeration systems various compressor cylinders are used with various types of liquid refrigerant lines, the pressure of the refrigerant delivered to the liquid refrigerant injection aperture may vary. Thus, in the design of a liquid injection system, sufficient pressure must be provided for the injected liquid refrigerant to account for the work performed during compression and re-expansion of the refrigerant within the liquid refrigerant path and by the pressure drop in the liquid refrigerant line so that the pressure of the liquid refrigerant exceeds the pressure within the compressor cylinder when the aperture is exposed. In the past, this has been accomplished by shifting the location of the aperture so that the pressure within the cylinder bore, whenever the aperture is open, is always greater than the pressure of the refrigerant within the aperture for any given liquid refrigerant line to be connected thereto. As discussed above, this decreases the amount of cooling provided to the motor when using refrigerant lines which can deliver a greater pressure and thus decreases the overall efficiency of the refrigeration system.
Another problem associated with the prior art liquid injection cooling methods is that the capillary tubes which have been used to provide the necessary pressure drop add to the overall cost of the refrigeration systems.